The present invention relates generally to glow plugs for diesel engines and, in particular, to glow plugs capable of measuring the ionization current inside the engine combustion chamber. The invention also relates to methods for manufacturing such glow plugs.
European published patent application number EP-A-0989370 describes a glow plug provided with a tubular metal body and with a metal sheath electrically insulated from the tubular body. An electrical heating element is housed inside the sheath and is connected to a first electrical terminal. The sheath is made of metal material and is insulated from the tubular body by means of a pair of rings of ceramic material set at the opposite ends of the tubular body. The sheath is electrically connected to a second terminal consisting of a wire provided with insulating coating which is welded to the end edge of the sheath and is set inside the tubular body.
The known solution described in the document EP-A-0989370 presents a number of drawbacks due to the high number of components necessary for ensuring electrical insulation and gas tightness between the sheath and the insulating body. The fact that the solution according to the prior art envisages the use of ceramic rings for insulating the sheath from the tubular body can entail considerable difficulties and high costs in order to achieve the necessary gas tightness on the contact surface between the ceramic rings and the sheath. In addition, the dimensions of the ceramic rings render the application of this solution to plugs with small diameters, for example 4 mm or 5 mm, difficult. A further disadvantage of the known solution lies in the difficulty in obtaining the necessary tolerances of coaxiality and roundness between the sheath and the tubular body.
A general object of the present invention is to provide a glow plug of the type indicated above that makes it possible to overcome the drawbacks referred to previously.
According to one aspect of the present invention, there is provided a glow plug that includes a metal tubular body, a metal sheath carried by the tubular body, a heating resistor located inside the sheath, first and second electrical terminals, and a layer of insulating material applied on a portion of the outer surface of the sheath. The first terminal is electrically connected to the heating resistor and the second terminal is electrically connected to the sheath. The layer of insulating material is located on the sheath between the sheath and tubular body to thereby electrically insulate the metal sheath from the metal tubular body. Preferably, the insulating material is applied by plasma deposition with the sheath being attached to the tubular body by an interference fit of the sheath within a cavity in the tubular portion.
In accordance with another aspect of the invention, there is provided a method of fabricating a glow plug as described above in which the sheath and its terminals and heating resistor together comprise a heating element carried by the tubular body. The method includes the steps of depositing a layer of insulating material on a portion of the outer surface of the sheath and fixing the heating element to the metal tubular body with the layer of insulating material disposed between the heating element and tubular body. Again, the insulating layer is preferably applied by plasma deposition and the sheath can be attached to the tubular body by forming an interference fit between the heating element and tubular body.